use executeAudited, add table name to assert message, skip schema changing test on oracle
correctly handle error results of PDO and MDB2 backends
remove wrong check here
isError should detect a PEAR_Error even if the backend is PDO.
This can happen on errors during schema migration - which is always done with MDB2
Going from text to clob is not something we do.

Also Oracle DB has problems with this, see
http://abhijitbashetti.blogspot.de/2011/10/converting-varchar2-to-clob-and-clob-to.html